(1) Field of the Invention
The present invention relates to a shock absorber controller for altering a damping force of a shock absorber in a vehicle suspension in response to a vertical movement of a vehicle body due to a rough road surface.
(2) Description of the Prior Art
Conventionally, there is known an apparatus for controlling a damping force of a shock absorber provided between a vehicle body and each wheel so that a vertical movement of a vehicle body due to a rough road surface is suppressed in an early stage of the vertical movement. Japanese Laid-Open Patent Application No. 62-166104, entitled "SHOCK ABSORBER CONTROLLER", discloses an apparatus in which the shock absorber is controlled so that the damping force thereof is altered from a weak state to a strong state when a predetermined delay time has elapsed after the vehicle height exceeds a definite value. The predetermined delay time is determined, taking into consideration the vertical movement cycle of the vehicle body so that the magnitude of the damping force is changed within half of the first one-cycle of the vertical movement. With this arrangement, it is possible to suppress a large vertical movement of the vehicle body within half of the first one-cycle so that the vehicle height can be restored to a mean vehicle height or position.
However, there is a problem that the timing of altering the magnitude of the damping force changes, depending on the magnitude of the vertical movement of the vehicle body. This will be described with reference to FIG. 1, in which the horizontal axis represents time and the vertical axis represents the vehicle height. When the vertical movement of the vehicle body has a relatively small waveform S.sub.1 of vibration, the vehicle height exceeds a predetermined height h.sub.0 at time t.sub.1 and the magnitude of damping force is changed at time t.sub.2 when a delay time t.sub.a elapses from time t.sub.1. On the other hand, when the vertical movement of the vehicle body has a relatively large waveform S.sub.2 of vibration, the vehicle height exceeds the predetermined height h.sub.0 at time t.sub.3 and the magnitude of damping force is changed at time t.sub.4 when the delay time t.sub.a elapses from t.sub.3. It will be seen from the graph of FIG. 1 that the magnitude of damping force is changed at different times for the different magnitudes of vertical movement of the vehicle body. That is, the altering of the damping force is not in synchronism with the vibration cycle of the vertical movement and is altered at a variety of different times. Thus, in some cases the magnitude of damping force is changed in a state where the vehicle height is considerably higher than the mean height or the vehicle height has almost returned to the mean height. Such altering of damping force deteriorates ride comfort.